Mn K-Edge X-ray Absorption Studies of Oxo- and Hydroxo-manganese(IV) Complexes: Experimental and Theoretical Insights into Pre-Edge Properties

Abstract

Mn K-edge X-ray absorption spectroscopy (XAS) was used to gain insights into the geometric and electronic structures of [MnII(Cl)2(Me2EBC)], [MnIV(OH)2(Me2EBC)]2+, and [MnIV(O)(OH)(Me2EBC)]+, which are all supported by the tetradentate, macrocyclic Me2EBC ligand (Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). Analysis of extended X-ray absorption fine structure (EXAFS) data for [MnIV(O)(OH)(Me2EBC)]+ revealed Mn–O scatterers at 1.71 and 1.84 Å and Mn–N scatterers at 2.11 Å, providing the first unambiguous support for the formulation of this species as an oxohydroxomanganese(IV) adduct. EXAFS-determined structural parameters for [MnII(Cl)2(Me2EBC)] and [MnIV(OH)2(Me2EBC)]2+ are consistent with previously reported crystal structures. The Mn pre-edge energies and intensities of these complexes were examined within the context of data for other oxo- and hydroxomanganese(IV) adducts, and time-dependent density functional theory (TD-DFT) computations were used to predict pre-edge properties for all compounds considered. This combined experimental and computational analysis revealed a correlation between the Mn–O(H) distances and pre-edge peak areas of MnIV=O and MnIV–OH complexes, but this trend was strongly modulated by the MnIV coordination geometry. Mn 3d-4p mixing, which primarily accounts for the pre-edge intensities, is not solely a function of the Mn–O(H) bond length; the coordination geometry also has a large effect on the distribution of pre-edge intensity. For tetragonal MnIV=O centers, more than 90% of the pre-edge intensity comes from excitations to the Mn=O σ* MO. Trigonal bipyramidal oxomanganese(IV) centers likewise feature excitations to the Mn=O σ* molecular orbital (MO) but also show intense transitions to 3dx2–y2 and 3dxy MOs because of enhanced 3d-4px,y mixing. This gives rise to a broader pre-edge feature for trigonal MnIV=O adducts. These results underscore the importance of reporting experimental pre-edge areas rather than peak heights. Finally, the TD-DFT method was applied to understand the pre-edge properties of a recently reported S = 1 MnV=O adduct; these findings are discussed within the context of previous examinations of oxomanganese(V) complexes.

High-spin Mn–oxo complexes and their relevance to the oxygen-evolving complex within photosystem II

“…(Table S3). Our computational results also agree with those reported recently using time-dependent density functional theory (TD-DFT) methods (25). Our results also show that the Mn-oxo bonds in the series of [Mn n H 3 buea(O)] m complexes are best described as having a formal bond order of 2, in which an empty z 2 orbital and two halffilled xz and yz orbitals are involved in the bonding.…”

Structural changes correlated with magnetic spin state isomorphism in the S2 state of the Mn4CaO5 cluster in the oxygen-evolving complex of photosystem II

“…S5 and Table S1 in the ESI†). 71 While the slightly larger pre-edge area observed in HS S 2 may indicate a more distorted ligand environment in this state as compared to LS S 2 , the difference is rather small for drawing any concrete conclusions.…”

Geometric and electronic structure of a peroxomanganese(iii) complex supported by a scorpionate ligand

“…The Mn K-edge X-ray absorption data of [Mn II (Tp Ph2 )(DMF) 3 ](OTf) shows a rising edge at 6547.5 eV, characteristic of a Mn II species, 62,63 as well as a pre-edge feature at 6540.6 eV (ESI, Figure S6). The Fourier transform of the extended X-ray absorption fine structure (EXAFS) data of [Mn II (Tp Ph2 )(DMF) 3 ](OTf) exhibits a primary peak at R′ = 1.7 Å which was best fit with a shell of 3 O atoms at 2.16 Å and a shell of 3 N atoms at 2.28 Å (Figure 8, Table 6).…”